While it is acknowledged that, roughly classified, there are 4 types of serotonin (5-HT) receptors, the neuronal serotonin (5-HT) receptors located in the primary afferent nerves of the enteric nervous system or of the central nervous system are considered, as of this day, to be 5-HT.sub.3 receptors. Regarding such 5-HT.sub.3 receptors, many compounds having 5-HT.sub.3 receptor antagonist activity have so far been discovered. For example, the compounds described in British Patents 2,125,398, 2,166,726, 2,126,728 and 2,153,821 are known to have such activity. However, there has not been discovered a compound having selective 5-HT.sub.3 receptor agonist activity.